Is a large UV map heavy on performance while playing?
A UV map is a set of coordinates representing the vertices of a mesh in 2D space. It is not usual to talk about it in terms of 'large' or 'small' as its size/density mostly depends on the size/density of the mesh. You should generally not care about resources used by UVMaps. The size of textures (like lightmaps) or the resolution of the whole mesh are usually the things you should worry about.